How can I put a higher value in #246 (HBP repayment) than I contributed in line #245 (RRSP contributions)?

You are trying to designate a higher payment towards your HBP than you actually have available in RRSP contributions?

Correct.  CRA confirmed this is how you do it.  I owe $621 dollars in HBP repayment.  I contributed $300 to RRSP for tax year.  CRA tells me to put $300 on line #245 (RRSP contrib), put $321 on line #129 (RRSP income, T4RSP), and then I can put $621 on line #246 (HBP repayment).  The only problem is TurboTax won't let me put a higher value than #245 in #246.

CRA told you wrong.  You can only show $300 as repayment...the software will automatically show $321 at line 129.  CRA will indicate the payment of $621 on their end, and it will be reflected on your Notice of Assessment.

Thank you.  I just confirmed with another CRA agent that indeed I should put $300 on #245, $300 on #246, and $321 on #129.  Thank you.

To be fair, the first CRA agent was likely thinking that at their end it shows as the full amount repaid, either way.

Can you tell me what you mean when you say the software will automatically show $321 at line #129?  That implies i need to have entered $621 *somewhere*, which I haven't.  Either way I think  my problem is solved as per above.

you have entered that the required payment is $621?

Ah gotcha, I understand now, I hadn't put in the required but I do see where to do that.  Thanks!

So when entering your information regarding repayment of the Home Buyer's Plan (HBP), you indicate how much the required repayment is, as well as your RRSP contributions for the year.  You can only enter up to the amount of RRSP contributions as repayment, so if your required repayment exceeds the amount available, the software will calculate the difference and apply the difference as income at line 129 of your tax return.

http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/rrsp-reer/hbp-rap/rpymnts/menu-eng.html