Try to position it using absolute position
.inner {
width: 50px;
height: 50px;
background-color: green;
position: absolute;
left: 100px;
}
.sidebar {
height: 300px;
width: 50px;
display: inline-block;
position: relative;
overflow: hidden;
/* left: 565px;*/
/*position: relative;i*/
margin:0 0px 0 10px;
}
Demo: Fiddle