sending an order through BTC-e API incorrect field error
The exact problem you're facing regards the number of acceptable decimal digits in the rate field.

You're trying to submit an order on the BTC/USD pair, which happens to have a max decimal allotment of 3. Meaning, you can submit an order for example 800.123, but NOT 800.1234 Not all pairs are the same. LTC/USD can have 6 digits, most BTC based pairs can accept 5 digits.

To get a list of exact information regarding the individual pairs, use this public endpoint for a JSON object that gives you all the values:  https://btc-e.com/api/3/info

As far as I know, all pairs' amount fields can have a max of 8 decimals. You are rounding both the amount and rate to 4 digits. Fix the rate field and it should work.

Please keep in mind when I say "decimal places" I mean actual number of digits after the period in a number. Not the numerical type decimal that you are using. Using a double or float will work as well, provided they are rounded properly.