Is the prayer of one who belched during rukoo' (bowing), and a small amount of liquid resembling vomit came to the middle of his mouth, so he expelled and wiped it away, valid?

Regurgitation (Qals) - which is what comes out of the stomach during burping - is pure (tahir) and does not nullify ablution (wudu'), even if it resembles vomit. Sheikh Ibn 'Uthaymeen clarified that vomit is not impure (najis), and the proof for this is the Prophet's (peace be upon him) silence regarding the command to wash off anything that came into contact with it. If this is established for vomit, then it applies to regurgitation even more so. The view that regurgitation is pure and does not nullify ablution is the more preponderant opinion. It is worth noting that it does not invalidate the prayer, unless something from it is swallowed intentionally.