Put money in thy purse
Iago:
I have profess'd me thy friend, and I confess me knit to thy
deserving with cables of perdurable toughness. I could never better
stead thee than now. Put money in thy purse; follow thou the
wars; defeat thy favor with an usurp'd beard. I say put money in
thy purse. It cannot be long that Desdemona should continue her
love to the Moor—put money in thy purse—nor he his to her.
Despite Iago's admission that "I am not what I am" [see HEART ON MY SLEEVE], Roderigo continues to step into all Iago's snares. Roderigo is frantically in love with Othello's wife Desdemona, a situation Iago skillfully exploits: he takes Roderigo's gifts, insisting that he's delivering them to Desdemona, while keeping them himself. Whenever Roderigo grows desperate at his lack of success, Iago has to revive the gull's impossible hopes, as he does here.
Insisting that he is bound to Roderigo by the indestructible cables of true friendship, Iago self-servingly orders his dupe to keep putting money in his purse. His other advice—to become a soldier, and to get a man's beard even if he has to get a phony one—merely marks time between the repetitions of "put money in thy purse." Iago is an absolute materialist, and—so that he may continue to line his own pockets—he frames Roderigo's prospects in material terms: there's no love that money can't buy.