What should I do about selling a device given for safekeeping and the denial of its receipt, knowing that the company has closed and its owner cannot be reached: Should I donate a small amount or the full amount, or am I not sinful?

It is not permissible to take someone else's property without right. Whoever does so must repent and return the right to its owner. If the questioner has received their full entitlements, then they must return the value of the "tablet" to the company owner. If they cannot reach him, they should give its value in charity on his behalf.