I don’t consider Inter Milan returning for Onana as a wise decision after selling him out to Manchester United, not even that he has impressed with Manchester United in his stay so far so I don’t see reasons to want Onana back as number one goalkeeper choice for Inter Milan next season there are other choices in the market, Martinez from Aston Villa is also available this summer.
Inter was not a dumb team. They would rather get Martinez from Villa instead of Onana. They successfully scammed MU with Onana's transfer. Why do they need him back? The better to let him stay in MU while they approach a new, cheaper, and capable goalie.
Furthermore a return to his parent club is extremely difficult unless the player is loaned out. Considering Onana was bought for €51 million, Manchester United would have to accept a loss by lowering his market value. Meanwhile Inter are not interested in bringing him back, so the best solution for Onana is to make next season a season where he can develop. If he continues to perform poorly, prepared to be released.