No-Limit
Qualification live MTT for a bigger tournament, but there is a substantial cash prize pool for this tournament. Late tournament, 12 players left out of 70, 7 get paid and a spot in the finals. Blinds 1500-3000. MP solid player limps, SB calls, I check in the BB with 35o. SB will see every limped flop when he is in the SB. He will take stabs at pots, but when the betting gets big he will lay his hand down, but he has been known to go too far with top pair. He had moved all-in a few times with medium hands preflop and has gotten lucky when called.
Stacks after posting blinds:
SB stack-29K
My stack-35K
Pot=9000
Flop 246 two spades one club
SB checks, I bet my made straight 5000, MP folds, SB calls.
Pot=19000
Turn As
SB goes all-in for 24K more
Total pot=43K
My odds=1.8/1
If I call, I will have 73K, and final table average will be 40K. If I fold, I will have 6K left and my tournament will essentially be over.
I honestly put him on a flush. He limped so he could have anything, he called my flop bet, then moved in with the flush card. The pot was big enough that he probably wanted to take it down right away. I thought the Ace helped him by pairing him, or he had the flush. I felt more strongly he had the flush. He had moved all-in preflop a few times, but I gave him more credit since he was all-in on the turn. I should have bet more on the flop, but I wanted some action and a little more than half pot bet seemed right.
This tournament is somewhat like a satellite, but there is incentive to play for first because there is substantial money to be won in addition to the seat in the bigger tournament, but playing for 7th first I think is the best. Even in a regular tournament this would be a tough decision.
What would you have done?
I folded and opponent showed A4 diamonds for 2 pair.
I felt sick.
I did go on to finish 7th and this opponent went out in 9th.
Thanks for the help.

Bit of Advice: If you want honest feedback in future you shouldn't post the results of the hand as it will influence people into saying here that you should have called.

People would argue that if he had the flush he is more likely to slowplay and check to you than to just shove all in like that. Very tough call to make though.

Yeah, I was thinking I shouldn't have posted the result, I will remember that for the future.

The pot already represented more than half his stack, in fact it was most of his stack so I figured that he was happy taking the pot down now and if he gets called it would be a bonus for him. Plus, because he limps in every pot that he is the SB, I certainly didn't put him on the nut flush so it is possible he felt I had a 4 flush and he was worried that if I caught a flush it would be better than his.

Thanks for your response.