For current annual income, what the I-864 requires is the income you will use to meet the requirements for sponsorship. To use income to qualify for sponsorship, you must establish your ability to maintain that income. Since you have no offer letter to continue working for the same company after you move to the US, you have no evidence that you can maintain that income. That's why your qualifying income should be entered as $0 in part 6, item 7 of the I-864.
Whatever the current income, you are required to enter (in part 6, item 24) the actual income figures reported in your federal tax filings for the 3 most recent tax years. There is no calculation required here. You simply copy the "Total Income" line in your tax return transcripts or 1040 forms. If you filed using form 1040 EZ, copy the line for adjusted gross income instead.
You can use assets alone to qualify for sponsorship if you have liquid assets valued at least 3x the income threshold. That's around $66,000 for a household of 2.
Note that it is the consul officer, not NVC, that has authority to decide whether you have sufficient assets to qualify or not. Be aware that the CO may still require a joint sponsor even if you listed enough assets to meet the minimum requirement. Typically, this is not something to worry about if your total assets are valued well above the minimum. VJ members have reported success sponsoring based solely on their assets.
