Trade War

I think both 'conditional + past' or 'future + present' are valid.

As Matt says because it is relayed or reported you can pass the original into the past.

One of my customers told me he wouldn't do business with our company until the trade war between America and China ended.

or as Jeff says 'had ended'.

but because the content refers to a future/ or permanent situation you can also use future + present.

One of my customers told me he won't do business with our company until the trade war between America and China ends.

or as Jeff says 'has ended'.

Trade War

Tense works out here because your statement is future tense embedded in a past tense narrative. If he were to be relaying this message to you right now, he would say, "I won't do business with your company until the Trade War between China and America ends." However, since you are relaying his message from a future perspective, won't changes to wouldn't, ends changes to ended. (tense stays the same throughout the sentence - told, wouldn't, ended) The only time this would change is if you quoted him, then you would say: "One of my customers said to me, 'I won't do business with your company until the Trade War between China and America ends.' "

P.S. What kind of farmer?