Is a person who received money to donate it, then used it for himself with the intention of repaying it later, and indeed repaid and donated it as instructed, considered sinful?

If you took money entrusted to you for charity without the owner's permission, you have sinned and must repent. This involves abstaining from such actions, regretting them, and resolving not to return to them, while spending the money you took on the purpose for which it was entrusted.