じゃ means then, well then. But what does じゃ in the above context? Seems the definition of then, well then can't fit in this sentence.
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This じゃ is a dialectal copula (linking verb) which is used instead of だ. This じゃ is actively used in some areas of western Japan (especially in Hiroshima), but it's also known as a typical role word of old men. You will also see じゃから instead of だから, じゃが instead of だが, じゃった instead of だった, and so on. じゃあ in modern standard Japanese is totally different.
If I bought a watermelon, both he and I like it, so it would be win-win.
で after a clause can sometimes denote a cause or a reason.