Why did Abraham give tithes to Melchizedek in recognition of his priesthood?
A tithe is an obligation, not a freewill choice to give thanks.
The biblical Messiah is an everlasting priest of the same kind as Melchizedek.
“The Lord said to my Lord, thou art a priest forever after the order of Melchizedek”
The Islamic Messiah is not.
Why does the institution of the priesthood vanish in Islam?
Because Mohammed is the new mediator?