Fragment Of A Medieval Dinar?

Not a dinar. That would be gold.

A dirham, silver. It is certainly arabic writing, early medieval I think. The first photo is upside down. Sorry I cannot say anything else from the fragment.

Islamic dirhams had very formulaic designs and legends, so it's often easy to recognize something as "definitely a dirham" but much harder to make a reasonable guess for the region or period. I'm not actually sure if there's even enough nonformulaic detail left to identify anything.
(There just might be. Unfortunately I can't read Arabic so I'm not sure what the remaining lettering actually says.)

Islamic silver coins circulated all over Europe in the Middle Ages - often, as this one probably was, cut up into pieces for bullion. AFAICT it is entirely no surprise to find one in northern Netherlands.
It looks like the coin had gone through quite a bit of wear before ending up underground, so it was probably pretty old by then already. Unfortunately this would probably also inhibit possible ID attempt further.

And yes, the first pic is upside down. The top line of the formulaic dirham legend is very recognizable.