Ask at Romanian National Bank; see the link. I'm not sure, but I suppose that in 1931 5 000 lei was the equivalent of a monthly salary for a worker.
1 Dollar (US) = 2,9932 lei (National Bank of Romania, today, 22.05.2009)
Check your stamp at the link below.
Today, 02.072011: 1 000 lei = 213 UK pounds
7 US dollers=209,524lei