Is expiation incumbent upon someone whose body touched the body of a woman who is a stranger to him (without undress) during the daytime in Ramadan, and is his fast for that day considered invalid?

A man touching a marriageable woman without necessity is forbidden for a fasting person and others alike, and it is even more forbidden for a fasting person, because fasting is a shield from sins and hellfire. Repenting from a sin is like one who has no sin. If something, be it madhy or maniy, is discharged from the man due to the touching, his fast is invalidated and he must only make up for it. If nothing is discharged, his fast is valid.