A tip:  ALWAYS use a white towel underneath any gun or item that you are disassembling, towels trap all parts that fall out and white color allows you to easily find them...

Going back to your problem: Don't get frustrated, those things happen to all of us sooner or later...Your issue is an easy one...

Get a "Strong Magnet" and pass it over the area where you dropped the detent ball...(Harbor Freight has these magnets for less than $5.00 See: https://www.harborfreight.com/66mm-round-magnet-96650.html

If this doesn't do it, consider removing the windage knob (use a towel as recommended above)...Both: elevation and windage knobs use the same ball bearing...Measure it with a caliper and if you don't find it where you live, email me at : azuaro at icehouse.net and I will gladly mail you some of the right size.

Note: Measure it in millimeters...The gun is German and all of their guns use the metric system. 

Best regards,

AZ
 
so the ball is the same size as the spring and or hole to which it goes, you can find metric bear balls on Ebay for a song and I'm betting it is a 2mm ball I could check for sure but not till tomorrow

https://www.feinwerkbau.de/en/Service-Download/Exploded-drawings

most come out of China and i get mine in Washington state in 14 to 30 days, always

5/64th would be a good choice also and again they can be found on Ebay

understand your gun so you know how they are made and what might pop out when your not thinking about it

it is an easy fix with a new ball, anyone should be able to do it
 
These ball bearings (link below) are 2.5 mm and are already in the US (don't know the manufacture origin)...VXB is one of our suppliers at the lab and they are a very good reliable company...They are made of CHROME STEEL vs. Stainless (Amazon link) and cost more ($5.37 x 250 bearings) but since there are already in the US you can have them sooner. The bearing that came in your gun is made of this material. 

Regards,

AZ

https://www.vxb.com/250-2-5mm-Diameter-Chrome-Steel-Bearing-Balls-G25-p/Kit12063.htm?gclid=CjwKCAjwnrjrBRAMEiwAXsCc4w38mAKNeEJ5ItZL8Z_oGTkYYMynnFV1KSu-TYIp82erbMGjJtZ6bxoCeGkQAvD_BwE