Why did ancient wills use Spanish dollar?

I was reading the Will of Cauder Mohideen (Kader Mydin) who founded the Masjid Kapitan Keling in Penang. His Will mentioned Spanish dollars instead of British currency. I was wondering why that was so. Today I discovered that the Spanish silver dollars were preferred as they contained higher quality silver.

Evolution of Currency:
http://www.mas.gov.sg/en/Evolution-of-Currency