Claiming fees from a sub-agent based on a verbal agreement and Whatsapp transactions

Hello Sirs,

I have a travel agency based in Abu Dhabi and I have a sub-agent (travel agency) who failed to pay me the visas issued under their responsibility. We only have a verbal agreement and WhatsApp transactions.

We agreed that she'll pay after every 5 days for each visa Issued. 15 days lapsed and still, she doesn't want to pay and says their clients did not pay yet.

It seems that they are putting the burden of their client’s non-payment on us, which should not be the case. She doesn't pick up our calls or messages and she gets furious whenever we ask for payment. Each day is a grace period it seems.

Please advise if there’s any way I can demand the amount from them and force them to pay us as soon as possible.

They even continue doing business through other suppliers and left us pursuing them.