What is the ruling on selling a musical instrument – which he owned before guidance – to benefit from its price in paying off debt, giving charity, and expanding an Islamic library?

The possession and sale of musical instruments have been previously forbidden, and their price considered illicit gain, due to the Prophet's (peace be upon him) saying: "Indeed, when Allah forbids something, He forbids its price."

Therefore, what you did was incorrect, and you should have destroyed the guitar. If you were aware of the prohibition of selling it, then do not spend its price on yourself unless you are poor and in need, in which case you may spend from it according to your need and donate the remainder to public Muslim welfare. However, if you were not aware of the prohibition of selling it, then some jurists hold the view that you are permitted to benefit from its price.