Thank you for explaining. Now I understand. Yes, in the states it is a legband to identify fowl, so I guess we would use the term 'banding' instead of 'ringing'.
Totally off topic, but I think that the Netherlands are wonderful. Lucky me, I got to go to Amsterdam (and some tourist side-trips Delft and Keukenhof) and I loved the land and the people. Maybe because 400 and some years ago some of my ancestors came from Amsterdam to the "New Amsterdam" now New York.
Here's a thought. Regarding ringing.... Perhaps there would still be a way to get the ducks ringed. Here in the US we don't have the system you explained - sometimes I think it would be good if we did, but we are far more casual about our birds. When we put leg bands on sometimes they are just plastic and a different color etc. There are various sizes etc. WE can put the leg bands on mature birds. Would there be a way to take the numbers from the leg ring and glue them on a flexible plastic ring? you may have to snip or file it down and then get some kind of super glue or something to attach it to the plastic band so that particular duck would have the information needed. It would probably look suspicious for a show or something like that, but maybe would work for your own purposes or perhaps for one of your customers.
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so the band could go on even after the duck foot is bigger than the ability to slip the legband over the foot.
Some of the metal bands can be attached too -- so perhaps you could get the numbers needed put on a ring that could be attached to the leg directly and bypass the foot. ??
Now, turning to the woman who may have scammed you, as one person said, perhaps she does have cancer, she just found out, and she was 'in shock' about it. When there are just too many coincidences, the stories people tell begin to fall apart. First she forgot her wallet, next she suddenly gets cancer.
Perhaps you should contact her, and say since she has cancer, she must want to reduce the chores she has and the amount of work, so you will be happy to take the ducks back (provided there is an alternative way you can ring them and they would have value for you) to ease her burden and make life easier as she deals with her cancer.
And I agree that you proababy want to deal with her only when you have people with you, a friend or relative who will be there for you.
If none of that works, and there is no legal remedy -- And it's interesting to hear you talk about police there compared to here. IMO most places in the USA probably have about the same level of police support that you have over there, from what I've seen, but they will sometimes step in and help.
Also the idea of 'small claims court' -- even here, one could get a judgement in small claims court, but the judgement could be hard or impossible to enforce.
Not knowing how much time has elapsed, I would suggest that you do take some action that can be documented. Seems that there must be some remedy - Maybe Google could help you out -- Put in something like 'in Netherlands someone hasn't paid for livestock they got from me, what remedy'. (except of course translated if Google is in Dutch over there)......Here's what Google translate gave me:
in Nederland heeft iemand niet betaald voor het vee dat ze van mij hebben gekregen, welke remedie '
Maybe some ideas will be brought up -- There must be some way to get your euros.
or maybe your ducks.....
ETA-- I put the translation back in google translate to translate from Dutch to English -- and it said 'someone has not paid for the cattle they got from me" -- I guess it is something to chuckle about as it got lost in translation.