support maximum 8 digits for unit price field

dear team 

when creating a purchase invoice, normally users need to type in the accurate total invoice amount which is absolutely the same as the amount in the received supplier bill or fapiao (supplier from China). if there are small gaps, like 0.01 or 0.02 difference, users normally need to adjust the unit price to match the bill amount. 

but currently, the maximum digit of unit price is 4, the small difference 0.01 can't be adjusted using 4 digits unit price. if DEAR can support 8 digits, it works.