Weird tax liability situation ?

I am freelancer software developer and recently sold my software with following facts :

1). The "web browser addon" was open-source and Free for end-users.

2). The US company paid money for my loyal user-base.

3).  The "web browser addon"  was earning me regular income since past 4-5 years by voluntary donations (by Paypal) and advertising revenues.


My question is can I save indian income tax under "royalties" or any other scheme as I earned valuable foreign currency for my country. It is to be noted that the company did not paid for the software as it was pretty basic and open-source, they paid for my huge user-base.