(Sou wa tonya ga orosanai; “Wholesalers don’t give that kind of discount”)
Things won’t go the way you want. Things aren’t going to be as easy as you expect. A line that you give people when they expect more than is realistic. The image is of someone going to the wholesaler with inflated expectations of what kind of a discount they can get.
We begin with the adverb そう (sou), “in that way,” marked as the topic of discussion and set up with an implied contrast (to a different way) by the particle は (wa). The comment that follows begins with the noun 問屋 (tonya). The term is commonly translated as “wholesaler,” although historically it can also refer to private shippers of goods, warehouse managers, and to the warehouses themselves. 問屋 is marked by the particle が (ga) as the subject of a verb. And the verb in question is 卸す (orosu), which originally meant “to grate” but by extension came to refer to selling things at (discounted) wholesale prices.
Some versions may use the older negative ending 卸さぬ (orosanu).
Incidentally, 問屋 can also be pronounced toiya. In fact, you’d expect toiya to be correct, given that the root of this “spelling” is in the verb 問う (tou). It appears that tonya is the Edo-dialect pronunciation… but somehow it became standardized, and is the correct reading for this saying.
(“Zenninsha no koto wo kojinteki ni kirai dakara, keiyaku kaijo shiyou to shiteiru no kai? Sou wa tonya ga orosan zo.”)
[“So, what, just because you personally hate your predecessor, you’re trying to get out of the contract? It’s not that simple.”]