Deuteronomy 20:11

And it shall be, if it make thee answer of peace
Comply with the terms of peace offered:

and open unto thee;
the gates of the city and its garrisons, and deliver all into their hands:

then it shall be that all the people that is found therein;
some having made their escape before the surrender of the city:

shall be tributaries unto thee:
pay a yearly tax imposed upon them, as the Moabites sometimes did, and which was paid in lambs and rams with the wool, ( 2 Kings 3:4 )

and they shall serve thee;
not as slaves, or be in continual bondage and servitude; but upon occasion be called out to any public service, as joining them against their enemies, rebuilding palaces and cities, or repairing walls of cities, and the like; and in general acknowledge their dominion over them, and their own subjection to them, by paying an annual tribute, or sending gifts unto them; thus the Moabites, Syrians, and Edomites, became the servants of David, ( 2 Samuel 8:2 2 Samuel 8:6 2 Samuel 8:14 ) .