Must I pay $300 to the internet cafe owner who lied about the slow internet and sold the line to his neighbor? And must the payment be at the current dollar rate if the amount owed was $300 at that time?

The agreed-upon rent, payable in dollars, must be paid. The fluctuation in the dollar's value against the Syrian currency is not to be considered, because both of you accepted the service as it was and agreed to benefit from it despite the defect of slowness.