Commerce Paypal Payment Standard Currency Conversion
The PayPal Payments Standard module only supports the currencies PayPal specifically identifies in its API documentation as supported currencies. When it encounters an unsupported currency, it converts it to whatever currency you have set as the default currency in the configuration of the related payment method rule. Since the default conversion rate is 1:1, that's why this is being passed to PayPal as 1500 USD.

I don't know to what extent PayPal supports the Dirham. If your account does, then it could be you need to patch the PayPal module to support payment transactions in AED. Otherwise you're going to need to use the Commerce Multicurrency module to set an actual conversion rate between the AED and the PayPal supported currency you want to receive payment / maintain a balance in.