“You say.

‘Whoever swears

By the altar,

Is bound by nothing.

But whoever swears

By the gift

That is on the altar,

Is bound

By the oath.’”

καί Ὃς ἂν ὀμόσῃ ἐν τῷ θυσιαστηρίῳ, οὐδέν ἐστιν· ὃς δ’ ἂν ὀμόσῃ ἐν τῷ δώρῳ τῷ ἐπάνω αὐτοῦ, ὀφείλει.

This unique diatribe against the Scribes and Pharisees continued in Matthew alone with the same idea and phrases that were expressed in verse 16.  These Pharisees and Scribes say that whoever swears by the altar (καί Ὃς ἂν ὀμόσῃ ἐν τῷ θυσιαστηρίῳ) was not bound by anything, because it was considered as nothing (οὐδέν ἐστιν).  However, anyone who swears by the gift that is on the altar (ὃς δ’ ἂν ὀμόσῃ ἐν τῷ δώρῳ τῷ ἐπάνω αὐτοῦ) was bound by his oath (ὀφείλει).  In other words, the only thing that they were bound to fulfill was the gift on the altar that they were contributing, not other vows or promises.  This goes back to the question of whether you should swear to do anything or not, as posed earlier in this work in chapter 5:33-37.