John Gill 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 your God blesses you, as he promised you
He is faithful that has promised, and he always gives the blessing he promises according to the nature of the promise; if absolute, and without conditions, he gives it without respect to any; but if conditional, as the promises of temporal good things to Israel were, he gives according as the condition is performed:

and you shall lend to many nations, but you shall not borrow ;
signifying they should be so rich and increased in goods, and worldly substance, that they should be able to lend to their neighbouring nations round about, but should stand in no need of borrowing of any of them. This is sometimes said of the language of these people, the Hebrew language, that it lends to all, but borrows of none, being an original primitive language; see (Deuteronomy 28:12)

and you shall reign over many nations :
which was fulfilled in the times of David and Solomon:

but they shall not reign over you ;
that is, as long as they observed the commands of God; otherwise, when they did not, they were carried captive into other countries, and other people reigned over them, as at this day.