score:6
In practice, none. Singapore Customs concentrates almost exclusively on contraband (drugs, cigarettes, etc) and a ring will not be of interest, even if its purchase value theoretically exceeds the duty free threshold (S$600). If you want to play it really safe you could always declare it at the red channel, but I suspect that unless your ring has the Cullinan diamond attached, you'll be waved on even if you do.
You may get some questions about how long you're planning to stay in Singapore and support yourself, but if you have a return ticket and strong reasons to return to Germany (job etc), this shouldn't be a problem either.