Currently, if your offer isn't good enough and the other person asks you to supplement your offer with funds to sweeten the deal, you will need to add funds on your own, we recommend doing this only at the time of the meetup or transfer using services like PayPal and Zelle however other services are available and cash is an option if no other method is convenient. Soon Swapnice will offer a service called "Swap Wallet" that will allow you to safely transfer some funds for the sake of the transaction.