SQL Syntax Help Group By type query

Hello all,

I have a table and loan as follows:

Table:  Loan

Fields:
LoanId, PoolId, RelID, RelSeq, Amount

I need a query that will do the following.  I am trying to get a list of 'primary loans' for a set of data.

The way the data comes together is as follows:

First Data Scenario:

I have a PoolId such as '123' and it may have 6 loans as follows:

LoanId, PoolId, RelID,  RelSeq,   Amount
1       123     1123-1  1         100.00
2       123     1123-1  NULL      200.00
3       123     1123-1  NULL      300.00

In this case I know I have a primary loan based on there is more than one RelId same value and I know that LoanId = 1 is the
primary based on the RelSeq = 1 of this group.

Second Data Scenario:

LoanId, PoolId, RelID,  RelSeq,   Amount
4       123     1123-2  NULL      100.00
5       123     1123-2  NULL      200.00
6       123     1123-2  NULL      300.00

In this case I know I have a group with a primary based on there is more than one RelId same value but the RelSeq are all
NULL in this case I need to take the 'greatest' amount value as the Primary so in this case LoanId = 6 is my Primary.  If for some reason the Amounts are all null or two the same Amount value just take the first.

I want to return a single result set with the two records there will be other records with RelID but not in a group just it's own
I don't want to return:

This would be my return set:
LoanId
1
6

Thanks all