You had been refusing to do combat training with Cassian for a while now, and it was getting progressively frustrating for him. So he decided to try what he wished would work.

"If you do an hour of exercise with me right now, I'll owe you a favour." He said to you, arms crossed over his chest as he gauged your reaction.

And since fae bargains were bound by magic, if you had agreed he couldn't go back on his word and that would make the deal settled with a tattoo. He was silently begging that this was an enticing enough offer.