50 eur was worth more then the loan amount as of when the loan was made (I am not exactly sure how the OP received the collateral though).

The lender should have had a stipulation that if the price of btc were to rise to a certain amount that the loan needs to be repaid or the collateral will simply be kept by the lender to cover the loan, and then used that 50 eur to purchase sufficient btc. Or when the loan became overdue, the lender should have used the collateral to cover as much of the loan as it could as of when the loan was originally due, especially if the borrower was no longer responding to PMs