to nourish a viper in one's bosom (do acts of kindness that will cause one harm in the end)

you scratch my back and I'll scratch yours

(informal) to earn a lot of money, to make a mint, fortune

a bird in the hand is worth two in the bush

(idiomatic) to have a hard nut to crack

one swallow does not a summer make

a person’s silence can be very telling; usually in the sense that it can be understood as a negative or declining answer

(idiomatic) to make a virtue of necessity (embrace something forced on one such that one pursues it to the full extent)

he who digs a pit for others falls in himself

(idiomatic) something has bitten him, something is eating him/something is eating at him (slightly different meaning; irritation without a specific cause: someone has gotten out of bed on the wrong side)