What is the ruling on selling musical instruments one owns after repenting from working with them? And is it permissible to sell them at a specific price to pay off their debt, knowing that they are not suitable for mosques and he needs them to face the burdens of marriage and unstable work?

It is forbidden to sell musical instruments and equipment, and they must be destroyed, except for what can be used for permissible (halal) purposes, in which case it may be utilized or sold to someone who will use it for permissible purposes. Regarding the debt resulting from purchasing these devices, try to convince the seller that he is not entitled to it because it is in exchange for a forbidden instrument. If he does not respond and you attempt to evade paying the loan, then do so. If you cannot, then seek another job to repay the loan, and know that whoever fears Allah—He will make for him a way out and provide for him from where he does not expect. And whoever gives up something for the sake of Allah, Allah will compensate him with something better.