that's a sharepoint limitation

see - List and library column types and options - SharePoint 

nintex implements hard limit to 15 digits for controls connected to list fields of type currency, resp. 30 digits to controls not conntected to list fields.

sharepoint itself, if you exceeds 15 digits, rounds up the number to 15 most significant digits

Hi Marian,

The SharePoint currency filed is allowing me to accept more than 15 digits.

Please find the below screen shot for your reference.

However the nintex form is not allowing me to enter more than 15 digits. Is there any solution to fix it.

I'm not aware of it would be possible to override nintex control limit.

the only way I can think of how you could workaround it is that you let users input the figure into regular single line text field (string) and then create a workflow that would copy over the value from string field into currency one.