the minimum salary is $3270 and not $3670. The $400 is a different issue and should be paid additionally by the employer to Immigration for tax. Normally the tax will be paid on a six month basis in advance ie $2400 to HKSAR as soon as the visa for the maid is approved by them.
Dont tell your maid that her minimum salary is $3670 per month which is wrong. Also, the amount of $3270 is also stated in your contract with them. You may have a look at it.