I have had deals cancelled a few turns after paying out a large sum. It's reasonably rare as I am careful when selecting trading partners. Check their integrity, their honour, and their alliances/wars. If they are an honourable clan and not diplomatically tied to a situation which may bring them into conflict with you (or even the "you are an enemy of my friend" status) then the gamble is safe enough to take. Also check their location - if they only have a single port and are at war with another clan, the province could be captured. That automatically breaks the trade agreement.
I only make the gamble when I can afford the loss. I start out with 1,000 koku and work my way up if the AI refuses. If they have not agreed by 5,000 then it is likely they will not agree unless I offer ridiculous terms. By the point I am needing to offer large sums as bribes, I am able to swallow a loss with no more than a shrug. Because I aim for a high per turn income, I replenish my treasury very quickly. More income means more flexibility. That is partly personal preference; it's not a hard and fast rule every player must use. Once I have around 6 provinces and a couple of trade nodes I find I am bringing in over 5,000 per turn after all of my expenses are paid, and sometimes as much as 8,000 or 10,000. That's on the higher difficulties so it will apply to legendary too. Coincidentally, it's around that point when I need to start paying people in order to get a trade agreement.
Based on what you have said, money will be lost regardless. If I offered a smaller sum over multiple turns and they broke the deal, I'd still have to pay. Cheeky AI! The small loss is a better deal - your 700 koku over 20 turns example adds up to 14,000 koku. Ouch!
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