Reference for Menachot 30:9

רבי אבא זוטי בעי לה הכי בעא מיניה ר' אלעזר מרב השוחט את הכבש לאכול כזית מחבירו למחר מהו חבירו כבש משמע ולא מפגל או דלמא לחם משמע ומפגל ליה

Surely one can apply here an a fortiori argument thus, If what helps to make the other piggul does not itself become piggul, then surely what cannot even help to make the other piggul does not itself become piggul! And do we apply an a fortiori argument of such a kind? Behold, it has been taught: It once happened that a man sowed [with his own seeds] his neighbour's vineyard which was in the budding stage, etc. But why?

Jastrow