‘ And the men answered her, “Our life for yours, if you do not tell about this our business, and it shall be, when YHWH has given us the land, that we will deal faithfully and truly with you.” '
Their pledge was a strong one, that their own lives might be forfeit if they failed (compare Ruth 1:17; 1 Kings 2:23; 1 Kings 20:10 for similar oaths). The condition was that she did not inform anyone about what they had been doing there, or what they had promised her. They were confident that YHWH would give them ‘the land', that is, in this case, that part by the Jordan. And when He did so they swore to show faithfulness and kindness and to be true to their promise.
LXX omits ‘if you do not tell about this our business' but compare Joshua 2:20 where LXX does have it. It may well be that LXX was ironing out repetitions.