Field BSEG-MENGE not editable in FI document

Hello all

i would like to ask you about the field MENGE in FI documetn on vendor invoice

the quantity is used on GL account line item and after the document is posted, a user would like to change this field.

I have set this in SPRO for all account types but still this field is not possible to change.

the vednor line is cleared though

is there a way we could change field MENGE after FI documetn is posted?

thansk for letting me know

Hana