[popillio]

Now i've been several times to thailand, so i know some about the local behavier, but one event i still don't get.
The story:
I invite some friends to have dinner with me. Usual the one that invite will pay the bill. Now when the evening comes to an end, one of my friends ask the bill and insist to pay it.
After a short discussion i couldn't bring him to other thoughts, so i decide to back off and let him pay.(don't want that he loose "face") I announced that the next time we meet each other the bill will be for me.
Few months later i invite them again and tell them i would take the check bill. Reply: we will see.
When that evening comes to an end i get the "kep satang" and payed it. Somehow they figure out how much i pay (400 bath/person) every one jumps into his wallet and they refund me 1200 bath. I refuge to take it. Finally it end up on the table in front of me.I replace it back to them, but they refuge to take it back,so it end up again in front of me...
Now what you suppose to do with that?
Finally i took it back (couldn't leave it on the table right?)
My gf said it was wrong to take it back.
I ask her: must i leave it on the table than? her answer: dunno.....big thip for the waitress....
So i'm kinda totally confused about this kinda custom.....can someone explane it to me how things work out?
How to behave in such situation?