If the deposit amount is stated in the rental agreement, and the rental agreement has a definite ending date, no changes in the deposit can be made unless both parties agree to the changes or the lease allows for changes.

All communications should be done via Certified Mail, Return Receipt Requested. 

Security deposits are attached to those whose names are stated within the lease, and are returned to the leaseholder(s) who has remained until the end of the rental term. This means that, for example, if three individuals share a unit as roommates, and two renters vacate prior to the end of the tenancy, the security deposit only has to be returned to the tenant who has remained. Of course, the landlord may choose to locate the tenants who had vacated, and remit to each individual the divided security deposit.

Minnesota law does not limit the amount a landlord may require as a security deposit.

A landlord can increase the amount of the security deposit at any time during a "periodic tenancy" (a rental agreement in which no final date is mentioned), but only if the tenant is given proper advance written notice. Generally, this is one rental period plus a day. Example.  If a monthly rental, then notice is 30 + 1 day or 31 days notice.

When you give the landlord a deposit, be sure to get AND keep the receipt EVEN if you give the landlord a check or money order. The amount you give should also be on the lease. Verify the correct amount  is on the lease.

At the end of the tenancy, the landlord must return the deposit to the tenant with interest.

Interest begins on the first day of the month following the full payment of the security deposit. Interest runs to the last day of the month in which the landlord returns the deposit. 

When a tenant has sued to recover a withheld deposit, interest would run to the day the judgment is entered in favor of the tenant.

However, some cities may have enacted their own laws with respect to this.

A landlord must return a tenant’s security deposit plus simple, non-compounded interest, (141) or give the tenant a written explanation as to why the deposit (or any part of the deposit) will not be returned. The landlord must do this within 21 days after the day the tenancy ends, provided that the tenant has given the landlord a forwarding address. 

If the landlord does not return the deposit or return an explanation in the time allowed, the landlord must pay the tenant a penalty equal to the amount withheld and interest and also pay the tenant the amount of the deposit and interest wrongfully withheld. 

Minnesota law allows a landlord to withhold from a security deposit only the amount necessary for unpaid rent , damages to the rental unit beyond ordinary wear and tear, or other money the tenant owes to the landlord under an agreement (e.g. water bills).

See the RentLaw.com Guide to normal wear and tear.

If a landlord does not return or transfer the deposit, the court may penalize the landlord $200 for each deposit not returned or transferred.