John Calvin Commentary Deuteronomy 15:6

"For Jehovah thy God will bless thee, as he promised thee: and thou shalt lend unto many nations, but thou shalt not borrow; and thou shalt rule over many nations, but they shall not rule over thee." — Deuteronomy 15:6 (ASV)

For the Lord thy God blesseth thee. He confirms the previous declaration, but ascends from the particular to the general. For, after having taught that they might expect from God’s blessing much more than they have bestowed on the poor, he now recalls their attention to the Covenant itself. This is as if to say that whatever they have is derived from that original fountain of God’s grace, when He made them inheritors of the land of Canaan.

God also reminds them that He then promised them abundant produce. He thus indicates that if they were stingy and ungenerous, they would cause the land to be barren.

When He says that they should lend to all nations, He speaks by way of amplification. This is also true in the next clause: that they should reign over the Gentiles. From this it follows that if there were any in want among them, it would arise from the wickedness and depravity of the people themselves.