FEMA

Sec 2(n)(i) of FEMA explain a foreign excahnge  means a "deposit, credit and BALANCE PAYABLE IN FOREIGN CURRENCY" covers a bill drawn in Dollars. Bill is  an authenticated document that acknowledge a debt or a balance payable .So a bill denominated in Dollar is a foreign Exchange too and drawn upon by a Resident another Resident in India is prohibited.

Also to ur second query ,even the payment made by purchasing co by Rupees to Selling Co bank A/C , the bank has no authority to convert those in to Dollar.Because the various A/Cs which credit or convert from $ to Rs or vice versa  (EEFC,FCNR etc) are regulated by FEMA that restricts some specific credit or debit to this A/Cs where your domestic receipt cannot be converted to Dollar.Read defination/regulation for EEFC and other such A/Cs in FEMA