Is it permissible to give a substitute banknote of the same value after entrusted money has mixed with personal money?

It was your duty to fulfill the trust without dereliction. If the entrusted item became mixed and it was impossible to return the exact item, it is permissible to give an equivalent of its kind. However, if you do not possess an equivalent, then you are obliged to pay its value at the price of the day of performance. For example, if it is impossible to return 50 dinars exactly, it is permissible to give an equivalent, and if an equivalent is not available, it is permissible to pay its value in another currency.