A and B were involved in negotiations. A was in a superior position, but there was one and new unfavourable condition that if B knew about, would put A in a huge disadvantage. This was kept secret and agreement has been made.
The story was casually retold in Kansai-ben:
それ、できるはずや。 「description of the unfavourable condition」のにそれをおくびにも出さずにやったからです。
I assume dialect is irrelevant here and the speaker would make it できるはずだ in hyōjungo. From the context I guess it is used to mean something like "it couldn't have failed" (because the disadvantage was kept secret) rather than "be able to".
What I am confused is the discrepancy in tense here: できた - はずだ - やった. Why wasn't it できるはずだった?
Is it a more general statement? Like "it always works out if you keep your mouth shut"?