Прижатие элемент к нижнему правому краю блока(с overflow-y: auto;)?

var btnBonusInfo = $(".btn_bonus__info"), 
  btnCloseBonus = $(".btn_close__bonus"); 
 
btnBonusInfo.on("click", function() { 
  $(this).parent(".bl_bonus").toggleClass("bl_bonus__active"); 
}); 
btnCloseBonus.on("click", function() { 
  $(this).closest(".bl_bonus").removeClass("bl_bonus__active"); 
});

.bl_bonus { 
  position: relative; 
  width: 100%; 
  max-width: 538px; 
  height: 444px; 
  border-radius: 6px; 
  margin-bottom: 40px; 
  background-color: #3c3f41; 
  border: 1px solid #d1c8ab; 
  box-sizing: border-box; 
} 
 
.bl_bonus__img { 
  width: 100%; 
} 
 
.btn_bonus__info { 
  left: 0; 
  top: 0; 
  width: 50px; 
  height: 50px; 
  border-radius: 0 0 50% 0 
} 
 
.btn_bonus__info, 
.btn_close__bonus { 
  position: absolute; 
  box-sizing: border-box; 
  border: 2px solid #555; 
  background-color: #3c3f41; 
  outline: 0; 
  z-index: 10; 
  cursor: pointer; 
} 
 
.btn_close__bonus { 
  top: initial; 
  left: initial; 
  display: block; 
  bottom: 0; 
  right: 0; 
  width: 30px; 
  height: 30px; 
  border-radius: 0; 
  z-index: 3; 
} 
 
.btn_close__bonus:after { 
  content: "X"; 
  position: absolute; 
  top: 50%; 
  left: 50%; 
  transform: translate(-50%, -50%); 
  color: #f00; 
  font-size: 30px; 
} 
 
.bl_bonus__upSide { 
  position: relative; 
  height: 100%; 
  min-height: 500px; 
  background: #09161f; 
  z-index: 4; 
} 
 
.bl_bonus__upSide:after { 
  content: ""; 
  display: table; 
  clear: both; 
  width: 100% 
} 
 
.bl_bonuses__page .bl_bonus:nth-child(1) .bl_bonus__upSide { 
  z-index: 5 
} 
 
.bl_bonuses__page .bl_bonus:nth-child(2) .bl_bonus__upSide { 
  z-index: 3 
} 
 
.bl_bonus__img { 
  display: block; 
  width: 100%; 
  min-height: 40px 
} 
 
.bl_bonus__wrapper { 
  width: 100%; 
  margin-top: 30px; 
  padding: 0 5px; 
  vertical-align: top 
} 
 
.bl_bonus__name, 
.bl_bonus__value { 
  text-align: center; 
  text-transform: uppercase 
} 
 
.bl_bonus__name { 
  position: relative; 
  display: block; 
  color: #fbe5b2; 
  font-size: 22px; 
  font-weight: 400 
} 
 
.bl_bonus__name span { 
  position: relative; 
  top: -4px; 
  padding: 0 16px; 
  background: #222 -webkit-gradient(linear, left top, right top, from(#222), to(#222), color-stop(.5, #fff)) 0 0 no-repeat; 
  -webkit-background-size: 150px; 
  color: rgba(255, 255, 255, .4); 
  -webkit-background-clip: text; 
  -webkit-animation: shine 3s infinite; 
  animation: shine 3s infinite; 
  text-shadow: 0 0 0 rgba(255, 255, 255, .5); 
  z-index: 2 
} 
 
.bl_bonus__name:after, 
.bl_bonus__name:before { 
  content: ""; 
  position: absolute; 
  top: 10px; 
  width: 134px; 
  height: 2px; 
  background-color: #fbe5b2; 
  z-index: 1 
} 
 
.bl_bonus__name:before { 
  left: 20px 
} 
 
.bl_bonus__value { 
  margin-top: 22px; 
  color: #fff; 
  font-size: 52px; 
  font-weight: 600; 
  letter-spacing: .13px; 
  line-height: 34px; 
  white-space: nowrap 
} 
 
.bl_bonus__motto, 
.btn_bonus__getBonus { 
  color: #fff; 
  font-size: 20px; 
  font-weight: 400 
} 
 
.bl_bonus__motto { 
  margin-top: 15px; 
  text-align: center 
} 
 
.btn_bonus__getBonus { 
  display: block; 
  width: 246px; 
  height: 54px; 
  line-height: 54px; 
  text-transform: uppercase; 
  border-radius: 10px; 
  margin: 10px auto; 
  box-sizing: border-box; 
  background-image: linear-gradient(to right, #fae53d 0%, #e88000 51%, #f6bd3a 100%); 
  border: 1px solid #fff 
} 
 
.btn_bonus__getBonus:focus, 
.btn_bonus__getBonus:hover { 
  color: #1d3559; 
  transition: .15s all ease-in 
} 
 
.bl_bonus__downSide { 
  position: absolute; 
  top: 0; 
  left: 0; 
  max-height: 442px; 
  width: 100%; 
  background: #09161f; 
  border: 2px solid #fff; 
  overflow-y: auto; 
  color: #a2a6af; 
  font-size: 14px; 
  transition: .5s all ease-in; 
} 
 
.bl_bonuses__page .bl_bonus:nth-child(1) .bl_bonus__downSide { 
  z-index: 4 
} 
 
.bl_bonuses__page .bl_bonus:nth-child(2) .bl_bonus__downSide { 
  z-index: 2 
} 
 
.bl_bonus.bl_bonus__active .bl_bonus__downSide { 
  top: 100%; 
  transition: .5s all ease-in; 
  z-index: 8 
} 
 
.bl_bonus.bl_bonus__active+.bl_bonus>.btn_bonus__info { 
  display: none 
} 
 
.bl_bonus__value.bl_bonus__downValue { 
  font-size: 52px; 
  line-height: 34px; 
  margin-top: 0; 
  margin-bottom: 15px 
} 
 
.bl_bonus__downSide p { 
  font-size: 14px; 
  line-height: 18px 
} 
 
.bl_bonus__downSide a { 
  color: #fdf1c7; 
  line-height: 20px; 
  text-decoration: underline 
} 
 
.bl_bonus__downSide h2, 
.bl_bonus__downSide h3 { 
  color: #fbe5b2; 
  font-size: 21px; 
  line-height: 23px; 
  text-transform: uppercase; 
  font-weight: 400 
} 
 
.bl_bonus__downSide ol li, 
.bl_bonus__downSide ul li { 
  padding-left: 10px; 
  margin-bottom: 10px; 
  color: #a2a6af 
} 
 
.bl_bonus__downSide ul li { 
  position: relative; 
} 
 
.bl_bonus__downSide ul li:before { 
  content: '•'; 
  color: #dfc00f; 
  margin-right: 10px 
} 
 
::-webkit-scrollbar { 
  width: 5px; 
} 
 
::-webkit-scrollbar-thumb { 
  border-width: 1px 1px 1px 2px; 
  border-color: #777; 
  background-color: #fbe2ae; 
} 
 
::-webkit-scrollbar-thumb:hover { 
  background-color: #173a56; 
} 
 
::-webkit-scrollbar-track-piece { 
  background-color: #173a56; 
} 
 
.btn_close__bonus_wrap { 
  text-align: right; 
  height: 30px; 
  margin-top: -30px; 
} 
 
.btn_close__bonus { 
  display: inline-block; 
  position: relative; 
  left: auto; 
  right: auto; 
  top: auto; 
  bottom: auto; 
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> 
<div class="bl_bonus"> 
  <button class="btn_bonus__info" type="button"></button> 
  <div class="bl_bonus__upSide"> 
    <img class="bl_bonus__img" src="https://via.placeholder.com/350x130" alt="bonus-ot-netgame-za-depozit"> 
    <div class="bl_bonus__wrapper"> 
      <p class="bl_bonus__name">Title for bonus</p> 
      <p class="bl_bonus__value">Value bonus</p> 
      <p class="bl_bonus__motto">short description about bonus</p> 
      <button class="btn_bonus__getBonus" type="button" data-href="#">Get bonus</button> 
    </div> 
  </div> 
  <div class="bl_bonus__downSide"> 
    <div class="bl_bonus_wrap"> 
      <p class="bl_bonus__value bl_bonus__downValue">Title2</p> 
 
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      harum odit iure adipisci, non quos, id, illo voluptate officiis! 
    </div> 
    <div class="btn_close__bonus_wrap"> 
    <button class="btn_close__bonus" type="button"></button> 
    </div> 
  </div>